Creating a Single Sign On VPN with Samba4 on Ubuntu/Debian Server

These instructions are pretty rough and were written before Samba AD was first released, but they "worked for me" and I hope they give others some guidance. I've tried to go into as much detail as possible (painfully so) but I'm sure there are things that I'm missing. Please expand upon this HOWTO if you do find errors.

Overview

1. The purpose of this guide, is to provide a step by step guidelines how to create a L2TP VPN server, which is fully integrated with the Samba4 Server.

Network Topology

2. Before going over how to actually build and configure the VPN server, we need first to understand our network topology. Basically our network consists of a Layer 2 switch, a Firewall Server (which in our case is also the network gateway), one Samba4 Domain Controller and one or more Linux/Windows client machines.

Install & Configure Your Samba4 Domain Controller

4. This guide assumes you have one/or more Samba4 Domain Controllers running on your network.
For the purposes of this guide, I will use "DC.Example.com" as our Domain Controller hostname and "Example.com" as our Domain Name. If you are unfamiliar with how to install samba4 on Debian/Ubuntu Server, please see Setting up Samba as an Active Directory Domain Controller.

Install & Configure a RADIUS Server

5. Once you have a Samba4 Server up and running, our next step is to install and configure a RADIUS Server as an alternative to the Microsoft IAS or NPS.
There are several Open Source RADIUS implementations. This guide uses FreeRADIUS.

Please note the that in our example the FreeRADIUS software is installed on our firewall server. It could also be installed on the domain controller or another server entirely.

The above configuration causes the RADIUS service to bind to our main network interface (eth0 in my example), and configures it to accept both authentication and accounting packets.

If you install this service on the Domain Controller, make sure to change the ipaddr to your DC's ip address.

8. Now we need to configure which clients can use the RADIUS service. This is done at the /etc/freeradius/clients.conf file. Please note that since in our example we have installed the FreeRADIUS on the Firewall server itself, the L2TP service which will define later connect to the RADIUS service via the local host, so basically there is nothing to do here except changing the default RADIUS client password.

However, if you have installed the FreeRADIUS server on the DC machine, then you will have to configure the FW server as a RADIUS client member:

client 172.16.0.1 {
secret = samba4
shortname = fw
nastype = other
}

It is also a good advice to define here an additional client for debugging purpose. We will use it later, once we will try to test and if our Radius Server can authenticate with the Samba4 domain controller.

9. Our next step is to disable the inner tunnel requests for EAP-TTLS and PEAP types on the RADIUS Server. This can be easy done by deleting the inner-tunnel file at the /etc/freeradius/sites-enabled folder.

sudo rm /etc/freeradius/sites-enabled/inner-tunnel

Eventually you should end up with the default site only, which should look like this:

10. Our last task is to configure the FreeRaidus modules. There are at least one relevant module which need to be configure, which is the LDAP module.

Let's start configure first the LDAP module. You will have to configure it and change the required parameters to reflect your own configuration (Like the identity parameter (the user that bind to the LDAP server), the basedn (which in our example is DC=Example,DC=Com).

The most important parameter is the access_attr = "msNPAllowDialin", which is the file that tells our RADIUS server, if the user is configure to allow a VPN access. The parameters change from TRUE to FALSE (case sensitive) via the ADUC Dial-in TAB:

11. If you want to use MS-CHAP or MS-CHAPv2 authentication, you will have to configure the winbind service on the firewall before continue. Please Note: that as for version 4.0.0alpha12-GIT-1a27343 of Samba4, the ntlm_auth command, which is required for MS-CHAP or MS-CHAPv2 authentication, is still under development. So if you installed your FreeRADIUS under the Domain Controller, you will not be able to work with MS-CHAP or MS-CHAPv2 authentication. MS-CHAP or MS-CHAPv2 authentication only work at the moment with Samba3 versions. For more info about the status of winbind with samab4 please see here

One more good tutorial by Charles Schwartz about integration FreeRADIUS to AD can be found here.

12. At this point you should be able to test if the FreeRADIUS is working with the samba4 ldap server. For testing proposes, please change one of your existing users to get "Allow access" permission under the Dial-in TAB. I can recommend on using VACMAN RADIUS Client Simulator from Vasco which can really help for testing our FreeRADIUS configuration. If you do plan to test it, please remember to update your /etc/freeradius/clients.conf file, and defining the new RADIUS client parameters. If everything goes fine, you will end up with this:

Or if you go with MS-CHAPv2

Install & Configure the L2TP Service

13. Our next step is to install a L2TP service. I personally prefer the implementation by the Xelerance Corporation, so this guide will assume you also will be using this service. (However, I would prefer to have a L2TP implementation that can support DHCP over IPSEC, but as far as I know it, no such software exists at the moment).

The Installation is quite simple using the apt-get engine exists in Debian/Ubuntu:

sudo apt-get install xl2tpd

Now we need to do some changes to /etc/xl2tpd/xl2tpd.conf configuration file:

All we did is basically providing the ip address of the L2TP tunnel, and the ip address range for the VPN clients. The PPP option file (pppoptfile) is also very impotent deceleration, since it is were we tell the L2TP server to use our configured RADIUS Server.

Please note that if you installed the Samba4 on the Firewall server, then MS-CHAP/MS-CHAPv2 authentication will not work, and you will need to use PAP authentication only, as following:

require-pap
refuse-chap
refuse-mschap
refuse-mschap-v2

One more thing you need to make sure about is the libradiusclient library. The xl2tpd service use the radius.so plug-in which exists in the ppp package. This plug-in depends on the libradiusclient library package. There are two libradiusclient packages exists in the Debian/Ubuntu repository, but I recommend to use the libradiusclient-ng2 package, with some minor tweaking which need to be done.